my 85 has 16/9.5 size rims now. Tire size 255/50. I want to upgarde to C6 Z06 rims in 17 inch size. The new rims will be 17/9.5 by 54 mm offset.Which size spacer would I require? The rear is very constricted as it is so I cannot see stuffing a larger tire in there. The 16 inch rims have backspacing of 171.35mm or 6.75 inches.The rims I have looked at are at "corvette wheels.com" They are the five spoke and I think will upgrade the 85 very sweetly. I also would like to know if the new tire size of 245/40 is good or can I get more rubber under there without tubbing the rear end?Anyone who has done this I would appreciate your help as I only want to order once. Thanks

54mm is close enough to the 56mm offset used on the 88+ cars so you need a 3/4" adapter guys use when fitting late C4 rims to early cars .
Factory fitted 9.5" on 89+ cars ( and 11" with 315 tires for that matter .50mm offset ; 7.9" backspacing on GS ) so tire clearance not a problem
245 too small for 9.5" rim .
Factory fitted 275/40/17 on 9.5" rims or 255/45/17 on 8.5" rims

thanks for the info . I am no tire shop guy and needed to know

my 85 has 16/9.5 size rims now. Tire size 255/50. I want to upgarde to C6 Z06 rims in 17 inch size. The new rims will be 17/9.5 by 54 mm offset.Which size spacer would I require? The rear is very constricted as it is so I cannot see stuffing a larger tire in there. The 16 inch rims have backspacing of 171.35mm or 6.75 inches.The rims I have looked at are at "corvette wheels.com" They are the five spoke and I think will upgrade the 85 very sweetly. I also would like to know if the new tire size of 245/40 is good or can I get more rubber under there without tubbing the rear end?Anyone who has done this I would appreciate your help as I only want to order once. Thanks

I think they're going to look really nice bro!

I think they're going to look really nice bro!
Can even do the 17" front / 18" rear combo
http://www.corvetteactioncenter.com/forums/978105-post10.html

I didn't think that size would fit?